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Hinako Satoh, Akita University (Japan)
Daizo Ishiyama, Akita University (Japan)
Toshio Mizuta, Akita University (Japan)
Masatsugu Yamamoto, Akita University (Japan)
Takanori Nakano, Research Institute for Human Nature (Japan)
Atsushi Ando, Research Institute for Human Nature (Japan)
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Tamagawa Hot Spring, which has precipitating Ra-Pb-bearing barite, is a hot spring in a volcanic gas-bearing hydrothermal system associated with activities of Quaternary andesitic to dacitic magma beneath Yakeyama Volcano. The geology around the hot spring consists of Miocene Aniai Formation, Quaternary Pre-Yakeyama lake deposits and Quaternary andesitic to dacitic volcanics of Yakeyama Volcano. Tamagawa Hot Spring is located in a volcanic crater formed by eruptions about 5000 years ago.
Thermal water in the Tamagawa Hot Spring is divided into three types: Cl-SO4-type (Ohbuki hot spring: pH 1.2), SO4-type (pH 1.8 to 2.9) and neutral type (pH 6.1). These hot springs occur from the center to marginal parts in the order of Cl-SO4-type, SO4-type and neutral type. Concentrations of Na, K, Mg, Ca, Fe, Mn, Rb, Sr, Ba, Pb, total REE, Th and U in the Ohbuki Cl-SO4-type thermal water are 52, 50, 49, 177, 170, 4, 0.23, 0.42, 1.19, 2.41, 0.32, 0.009 and 0.005 ppm, respectively.?The concentrations of dissolved components of the Ohbuki Cl-SO4-type thermal water are about ten-times higher than those of components of the SO4-type thermal water. Data of Cl, SO4 and F concentrations and hydrogen and oxygen isotopic ratios of the Cl-SO4-type thermal water suggest that Cl and some water in the thermal water is derived from magma.
The Sr isotopic ratios range from 0.703992 to 0.704032 for the Ohbuki Cl-SO4-type thermal water. The Sr isotopic ratios of the Cl-SO4-type thermal water are lower than those of Yakeyama andesitic volcanics (0.704065 to 0.704093), Quaternary Pre-Yakeyama lake deposits and Miocene Aniai Formation (0.704130 to 0.707132) occurring below the hot spring area. On the other hand, the Sr isotopic ratio of the SO4-type thermal water around the hot spring area is 0.704245. The Sr isotopic ratio of SO4-type thermal water are in the range between Sr isotopic ratios of andesite and river water (0.704314 to 0.704519) in the Tamagawa hot spring area. Based on the data of Sr isotopic ratios and geology around the hot spring area, the SO4-type thermal water is thought to be formed by interaction between the andesite and groundwater derived from river water in the area. The idea regarding the formation process for the SO4-type thermal water is also supported by the facts that the SO4-type thermal water is formed by mixing of fumarolic steam including CO2 and H2S with shallow groundwater and oxidation of H2S at near-surface environments around the hot spring area. These facts suggest that the possible origins of Sr in the Ohbuki Cl-SO4-type thermal water and in the SO4-type thermal water are different.
The one of possible sources of Sr in the Cl-SO4-type thermal water is thought to be solidifying andesitic to dacitic magma, because it is expected that the magma has low Sr isotopic ratios. Components including rare metals in Cl-SO4-type thermal water could be also transported from deeper part to the surface of the earth.
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